After eating lots of falfels i think what makes a falafel taste good to me is to to make it intersting - so :

a very nice thing is a free salad take - they give you a pita with humus and falfels and you can add whatever you want

and than the more salds they got the better - i think that even falfel places with bad falfels can taste good if they have good saldes - and if they have a big variety than the salds can taste bad also